High frequency oscillators



June 28, 1960 R. DUNsMUlR 2,943,235

HIGH FREQUENCY oscILLAToRs Filed Dec. 18, 195e 2 sheets-sheet 1 FPOBER T DUNM U/R TTORNEW/ United States l Patent lO n "294323'5.' Y HIGH FREQUENCY oscILLAToRs :IRoblert Dunsmuir, Rugby, England, assignor to The Brit- Thomson-Houston Company Limited, London, England, a British company Filed Dec. 18, 1956, Ser. No. 629,106

priority, application Great Britain Dec. 19, 1955 8 Claims. (Cl. S15-39.75)

vconsisting of a series of segments defining resonant cavities ,electro-'magnetically coupled together. It is customary to use an even number of segments and to excite, by electron streams owng from a central cathode to the segments, oscillations in which the alternate segments are in phase opposition. This is known as the pi-mode.

number of maxima equal to the number of segments.

Other-modes of oscillation may occur in which the eld pattern has fewer amplitude variations and these modes occur .at different frequencies andare excited by electron streams, of dierent velocities; ,that is, diierent anodejcathode voltages give riseto oscillation in thesediterent `'I'he modes of a magnetron with N segments are described ,in terms of 4thefundamental fourier sine compo- Nnentof the RF electric eld pattern, the number of repeats. of this component being called a mode number and denoted nl Both standing wave and travelling `wave modes may occur, la standing wave comprising equal and Voppositely directed travelling waves. f g

The pi-mode can only occur for an even number of segments Iandvisa standing wave with substantially equal Iamplitude at the various resonators. AA single coupling loop orprobe ,atl any point in the resonator system can always" bemade to couple reasonably ltightly to the pi- 'modef (for the purpose of extracting power), and morelover,l the pimode can be excitedfby elect-ron streams rotatin'gabout the cathode in either direction, as determined by the direction of the D70. Ymagnetieiield. Y Modes other than the pi-mode (except yfor the nf-O or 2,943,235 Patented June 28, 1960 If the rotating electrons couple very lightly to theresonant system so as to have negligible influence on the propagation characteristics of waves around the resonator system, the presence of irregularities and losses in the resonant system determines whether travelling wave or .standing wave modes occur. j

Irregularities maybe due to non uniformities in the metal Wallsr of the cavity resonators. These can be made negligible by careful design and construction. An irregularity also occurs due to the output coupling probe Aor loop..g'y f i Power is, lost within the resonator system dueto the resistance of the resonator Walls and currents flowing in them.;V These losses are usually measured in terms of the Q factorV of theunloaded magnetron Qo, andthe coupling of the output loop isvdescribed by Qx. It canbe shown that steady state travelling Waves can only occur inthe "The amplitude of the electric field of the piLmode has a ,y

,.zerofmode) may occur either vas travelling waves or as standing waves. Travelling Awave `modes couple equally toa single output probe for all positions of theprobe. Standing wave modes, other than the pi-mode have dif- `lferent' amplitudes in the different resonators and in general have zero amplitude in at least two places. l outp put probe situatedat either of these nodes can extract nopower. Y. Y.. .s

The fonrier analysis of the iield pattern of a non-pimode has `a fundamental (lowest periodicity) component with less thanY N/2. repeats round .the system. There is also y a component of slightly smaller amplitude with A standing wave non-pi-mode can be excited by high velocity electrons coupling withY the fundamental components and rotating in either direction, and also by lower velocity electrons coupling with the N -n components' and rotating fin either direction. l y v- `resonantsystem is Qx is substantially greater than Q0 or if the electron stream is coupled tightly to the resonators. The first condition corresponds to very low output.

In practice it is diicult to arrange for a magnetron to operate always in the same mode. The pi-mode is most desirable because it is certainly coupled to the output, but there is always a tendency to operate in modes whose fundamental periodicity is (N/,Z-l). Other non-p'- modes; are mguch less troublesome. Y Y v Considering the pimode and Athe (N/ 2,-1) modes Ain, a magnetroninwhich no; particular means are provided to suppress or prevent excitation of waves travelling in one direction on the circuit','i;e. in a standing wavemag? --netr'om .thereiare-ten' possible types of operation as shown in'ITable tI.' In :this table the secondcolumngives the Vperiodicity that-would lbe measuredon the circuit'seg- ,ments-by -a :Suitable probe, and? the fourth column ngives Y `indicatedin the third column. c Y

' The ,electron rotation can vbe in either direction depending on the `direction of the magnetic eld as indicated invthejffthcolumn, andthe `interactingtravelling wave :fourier component must :necessarily be in the same direclion; M .j t' g I g'It willbe evidentthat with these variants thereg are tenupossible'combinations,in all. A l

Howeverbysuppressing or preventing excitation `of .waves travelling .inone ,or-.other. direction a' travelling wave` magnetron isproduced,A `and assuming that a .direction of, rotation ofvgelectronsrhas been chosenthe, possible Y types-0f '-opelat'llvareyreduced to three, asrshownfin fable ,Hr 'f 1.11 fTableS I 'and II N represents, the number of Selllliill U16 magnetron; 1 ,y TABLEI N `Tvltf 'fpeflin in standing .wave magnetron withieyren,

`v`number of segments A Y Fundamental Interacting Oomponent Coupling l i to Electron g; output Rotation clockwise. anti-clockwise. clockwise. v

anti-clockwise. y clockwise.v

anti-clockwise. clockwise. anti-clockwise. clockwise... g'. anti-clockwise.V f

The main object of the invention to eliminate, or at least to reduce to negligible proportions', unwanted modes of oscillation,

It has been found that m pulsemulti-resonator magnetrons it is possibleV to couple the electron streamsuffi- `ciently tightly to the resonators-for travelling waves to occur in the magnetron for low values of the'Q factor of the output coupling provided that no large irregularities, other than that due to the output coupling existing in the resonator system (i.e. ther tight coupling of the electron stream overcomes the disturbance produced by the output coupling).

To -achieve this it is important to have a uniform resonator system and avoid strapbreaks. l Y

It the resonator system has an odd numberof segments there is no Vpi-mode and` the4 number ofpossible types of operation is reduced to two namely types 12 and 13 of Table II (with components The choice of yan odd number of resonators to eliminate the pi-mode leads to certain problems with regard to coupling Abetween resonators. For example, the normal Atypes of strapping in which the connection is made to al- -ternate segments cannot be used.

The difficulties may be overcomeas follows: (l) By using continuous straps connected to alternate segments and each making two` completercircuits-round the resonant system so that each strap wire provides a closed loop of two turns.

(2) The number of resonators is made an odd multiple 'One or more circular ring straps are used, each ring Ybeing connected toV every third resonator. In a suitable application of this principle three circular ring anode block, each ring passing through clearance holes in the resonator walls and making electric contact with every third resonator, each ring contact being staggered Yby one resonator trom the neighbouring ring.

Vstraps are evenly spaced throughout the depth of the (3) The number of resonators is made an odd multiple of 3. No-straps are used` but, in analogy with the well knownRising Sun resonator system', every third resonator is made larger than its twozprecedingfellows.

Alternatively, or in addition tothe above arrangements the magnetron may be coupled -to the load through a directional coupler so that Vrotating waves in only one direction are heavily coupled and waves rotating in the Iopposite direction are lightly coupled. ln this. way standing waves may be substantially eliminated.

In order that the invention may be more clearly Vunderstood reference will now be maderto the accompanying drawings, in which:

Fig. 1 shows a magnetron having an odd number of segments. -Y

Fig. 2 is a view in a radial direction of a. segment of a magnetron of the kind illustrated in Eig. 1 andjshowsone form of directional coupler.

Fig. 3 is a diagrammatic ligure illustratingthe principle of operation of a so-called rat race.

Fig 4 shows. a magnetron embodying the 'fratjrace .g

ofFig. 3.

. Fig. 5 aperspective view, partly brokenaway, oi a strapped magnetron; and y of the wave on the transmission line.

Fig. 6 is a plan view of a magnetron having a rising sun resonator system.

Referring rst to Fig. 1, this shows a magnetron having an odd number of resonators R and segments S.

A strap P makes. two. complete encirclements of the magnetron coupling; the strap couples with alternate segments and is isolated from the intervening segments. The two ends of the strap. are coupled to the uppermost segment S1 so that the strap is eilectively endless,

By means of directional couplers it is possible to couple differently to clockwise and anti-clockwise rotating waves on the anode circuit. For example the (N1)/2 mode can be heavily damped and the (N+1)/2 mode lightly coupled to the output. This system then has only one eiective mode of oscillation.

Fig. 2 shows an arrangement which consists of a coaxial line L joined to a transmission line or waveguide by a coupling loop C. The two ends of the loop are connected respectively to the centre conductor of the coaxial line and through a series resistance R to the outer conductor of the line. An electromagnetic travelling Wave on the transmission line induces currents. in the loop in two ways. In the iirst place, the R.F. magnetic iield threads the loop and induces a voltage. In the second place, the displacement current, corresponding to the electric field (owing in the capacity between the tip of the loop and one of the conductors of the transmission line), is provided by charge ilowing in the same direction through the two halves `of the loop towards its apex. Thus, -the currents due to'E andA H assist in one half of the loop and oppose in the other half, or vice versa, according to the relative directions of E and H. These relative directions depend upon the propagation direction 0 Thus, for a wave travelling to the right, there may be little current iiow through R- but a large current flowing in the centre conductor of the coaxial line. On the other hand, the wave travelling to the left, there may be little current in the `coaxial line and a large current in R.` Since the characteristic impedance of the coaxial line and the resistance R'n'eed not have the same value, these two cases correspond to very dilerent loading of the wave.

An yalternative arrangement for producing dileiential loading of waves travelling round the magnetron circuit in the two directions is shown in Fig. 4. Thisis best understood by reference to Fig. 3 which shows a waveguide (or co-axial line) bridge circuit known as a "rat race. A wave entering branch 21is. divided between branches 22 and 24, no power leaving through branch 23. The proportion of power in branches 22 and 24 depends upon their relative impedance and coupling to the ring. Similarly, a wave entering through branch 24is divided between branches 21 and 23, according to their impedances.

Now referring to Fig.'` 4 the magnetron circuit corresponds to branches 21 and 24 joined together in a ring and to the lower portion of the rat race. -The coaxial line represents. the upper portion of the rat race and has unequal outlets, Z2 and 23. 'I'he coaxial line is W4, wavelength long'andthe portion of the magnetron circuitbetween 21 and 24also isti@ wavelength. The couplings of the coaxial line are substantially. matched to the resonators. I For this system, a wave travelling in the clockwise directionA is lightly loaded, a small amount of .power passing out through branch 22, whereas a wave travelling in the antifclockwise direction is heavily damped, a large proportion4 of .thepower owing out through branch 23. The clockwise wave suffers little reflection, .very-nearly all -thepower continuing through the lower half of the ra't race, i.e. round the magnetron resonant system.

By the directive coupling means describedabove, it `is possible to arrange that'waves travelling in `one direction round the anode systemareheavilydamped and. magnetron operation will only occur for waves travelling in the lightly loaded direction. Such waves may be excited by electron streams travelling in either direction according to whether these interact with a foward or backward space harmonic field component of the wave. Which of these actually occurs depends on the direction of the steady magnetic field.

In many cases it is advantageous to use the back-V ward space harmonic component since this leads to more rapid build-up of oscillations and to more uniform interaction between the electrons and the wave around the resonant system.

Referring now to Fig. 5, there is shown therein a magnetron anode block being divided by a number of segments such as S1, S2', S3, S4, etc. between which are defined resonators R1, R2, R3, R4 etc. The block contains three circular ring straps 11, 12 and 13 evenly spaced throughout its depth. Each ring passes through clearance holes, such as 14, in the resonator walls and makes electrical contact with every third segment. Each ring contact is staggered by one resonator from the neighbouring ring. Thus ring 13 contacts segments S1 and S4, ring 12 contacts segments S2 and S5 and ring 11 contacts segment S3.

Referring now to Fig. 6, there is shown therein an anode block having nine segments numbered S1 to S9 defining nine resonators numbered R1 to R9. No straps are used but in analogy with the well known rising sun system every third resonator R1, R4 and R7 is made larger than its two preceding resonators.

What I claim is:

l. A multi-resonator magnetron having an odd number of segments defining resonators which` segments are symmetrically positioned around the circumference of said magnetron, means for producing a rotating electron stream in the magnetron which interacts with the resonators to produce electromagnetic waves, and means for preventing said electromagnetic waves from travelling in one direction whereby to produce electromagnetic waves travelling in the opposite direction only round the magnetron.

2. A multi-resonator magnetron having an odd number of segments arranged symmetrically around the circumference of said magnetron and defining a corresponding number of substantially identical resonators, means for coupling each resonator tightly to both adjacent resonators, an output feeder to said magnetron, means for coupling at least one resonator to said output feeder to feed power out of the magnetron, means for producing a rotating electron stream in the magnetron, means for coupling said stream sufficiently tightly to said resonators to produce travelling electromagnetic waves rotating in one predetermined direction only.

3. A multi-resonator magnetron having an odd number of segments defining resonators arranged in a circular symmetrical pattern, means for producing a rotating electron stream in the magnetron which interacts With the resonators to produce travelling electromagnetic waves and means for damping electromagnetic waves travelling in one direction but allowing electromagnetic waves to travel in theA opposite direction.

4. A multi-resonator magnetron having an odd nurnber of segments defining resonators, means for strapping each segment to both alternate segments on either side of a segment and means forproducing a rotating elec- 6 tron stream in the magnetron means for tightly coupling said electron stream with said resonators to produce electromagnetic Waves and means for preventing said electromagnetic waves from travelling in one direction so that the electromagnetic waves travel in the 0pposite direction only round the magnetron.

5. A multi-resonator magnetron having an odd number of segments defining resonators, at least one continuous strap connected to successive alternate segments and making two complete circuits round the resonators as a two turn closed loop and means for producing a rotating electron stream in the magnetron and means for coupling said stream with the resonators to produce electromagnetic waves travelling in one direction only.

6. A multi-resonator magnetron having a number of segments defining resonators which is an odd multiple of three, three ring straps each connected to every third segment the connections being staggered so that each ring connects to a different one-third of the segments and means for producing a rotating electron stream in the magnetron and means for coupling said stream with the resonators to produce electromagnetic waves travelling in one direction only.

7. A multi-resonator magnetron comprising a cylindrical anode block having an odd number of segments evenly spaced around the periphery of said anode block,

means for producing a rotating electron stream in the magnetron which interacts with the resonators to produce travelling electromagnetic waves, and a directional Y coupler for extracting energy from the magnetron, and means whereby electromagnetic waves travelling in one direction are damped by the coupler lbut electromagnetic waves travelling in the opposite direction are lightly coupled by the coupler.

8. A multi-resonator magnetron having an odd number of segments defining resonators, means for producing a rotating electron stream in the magnetron Vwhich interacts with the resonators to produceV travelling electromagnetic waves, separate matched couplers located respectively in each of two resonators spaced apart Vby three-quarters of a wavelength in the magnetron at the required oscillatory frequency of the magnetron, an external transmission path having a length of three-quarters of a wavelength connected between said couplers, two outlets to said transmission path positioned symmetrically therein and separated from each other by a distance of a quarter wavelength, means whereby electromagnetic waves travelling in one direction couple to one outlet while electromagnetic waves travelling in the opposite direction couple to the other outlet, and Y means whereby one outlet is tightly coupled to said transmission path while the other outlet is` lightly coupled thereto so that electromagnetic waves travelling in one direction in the magnetron are damped, while electromagnetic waves travelling in the opposite direction are sustained.

References Cited in the lille of this patent y UNITED STATES PATENTS Olive Aug. 14, 1956y 

